DIYARBAKIR, Turkey (Reuters) - Kurdish militants detonated a car bomb near a police checkpoint in southeast Turkey early on Sunday, killing two police officers and wounding five others, security sources said, marking the latest casualties in the region's worst violence in two decades.

More than 100 police and soldiers have been killed, along with hundreds of militants, in renewed conflict since a ceasefire collapsed in July, shattering a peace process launched in 2012.

The sources said Turkish security forces at a nearby base in Sirnak province subsequently shelled a mountainous area to which the Kurdistan Workers Party (PKK) fighters had fled after the attack.

Two militants were killed in the operation, which was supported by Cobra attack helicopters. Sikorsky helicopters landed commandos in the area where the PKK fighters were and there were intermittent clashes.

PKK guerrillas also launched an attack on Sunday with rocket-propelled grenades and rifles in the Silvan district of Diyarbakir province, killing one police officer and wounding another, one security source told Reuters.

In the center of Diyarbakir, the largest city in the mainly Kurdish southeast, a curfew was declared early on Sunday in the historic Sur district, the provincial governor's office said in a statement.

Witnesses said gunfire could be heard in the Sur area but there were no immediate reports of any casualties. It was not clear when the curfew would end.

A week-long curfew in the town of Cizre, near the borders with Syria and Iraq, was lifted on Friday. A pro-Kurdish party has said 21 civilians were killed during clashes in the town. The government said one civilian and 32 militants died.

The PKK began its separatist insurgency in 1984 and more than 40,000 people have been killed in the conflict. It is designated a terrorist organization by Turkey, the European Union and the United States.

President Tayyip Erdogan has promised the fight will go on until "not one terrorist is left". The conflict has flared up as Turkey prepares for a snap parliamentary election on Nov. 1 after a June vote was inconclusive.

(Reporting by Seyhmus Cakan; Writing by Daren Butler; Editing by Catherine Evans)